A wide variety of techniques for the manufacture of semiconductor devices and semiconductor structures are known. These include, for example, techniques in which a layer, film, or some other body of material is formed over a body of semiconductor material. For numerous reasons it may be desirable to remove oxides from the surface of the body of semiconductor material before the body of material is formed over it. A general problem is how to remove such oxides without detrimentally affecting the semiconductor body surface. More specific problems will be appreciated from the following discussion.
Ex-situ pattering of GaAs substrates, combined with molecular beam epitaxy (MBE) regrowth, provides a technique for the wafer scale production of new designs of three-dimensional devices. However, to exploit fully the length scales available through MBE growth, and achieve patterning in all three-dimensions, it is necessary to regrow active layers in close proximity to the patterned interface. The production of a clean, smooth regrowth interface is therefore essential. Conventionally, cleaning of a GaAs surface prior to MBE growth is achieved by thermal desorption of oxide layers. Such cleaning requires temperatures in excess of ˜580° C. and an As overpressure, but can have limited effectiveness. This is because, at high temperatures, the most stable surface oxide, Ga2O3, reacts with the host substrate through the reaction Ga2O3+4GaAs→3Ga2O↑+2As2 (or As4)↑ to form the more volatile oxide Ga2O. Thermal cleaning of the GaAs surface, therefore, tends to result in a pitted surface owing to GaAs removal from the substrate. Such surface pits are considered to be the cause of the formation of macroscopic ‘mounds’ during the epitaxial growth of GaAs, and require planarization with a GaAs buffer layer of typically ˜0.5 μm thickness. This issue must be addressed to achieve the high quality interfaces required for fabricating nanostructures through regrowth on patterned substrates.
Recently, both hydrogen-assisted cleaning and gallium-assisted oxide desorption have been shown to reduce pit formation significantly, leading to flat, unpitted and oxide-free surface. This has led to the growth of site-controlled quantum dots (QDs) on patterned substrates, with reasonably good optical properties. Hydrogen-assisted cleaning allows gallium oxides to be removed at lower temperatures, ˜400° C., without surface pitting. However, this method is known to lead to Fermi-level surface pinning and possible degradation of the surface if the hydrogen dose is not precisely controlled. In addition, surface contamination could be introduced during the hydrogen-assisted cleaning process, if extreme care is not taken. Gallium-assisted desorption removes native GaAs surface oxide at temperatures above ˜420° C., in the absence of an arsenic flux, through the reaction Ga2O3+4Ga→3Ga2O. Compared with hydrogen-assisted cleaning, gallium-assisted desorption has the practical advantage that the desorption can be carried out in the growth chamber, with the oxide removal being monitored in-situ by reflection high-energy electron diffraction (RHEED). Furthermore, there is no requirement for additional apparatus, such as a hydrogen source and associated turbomolecular pump, or a dedicated cleaning chamber. However, the technique is very sensitive to the precise amount of oxide on the surface. Sub-monolayer precision of the gallium flux is required to remove the oxides effectively, but avoid droplet formation and the infill of pre-patterned holes. This is a direct consequence of the lower vapour pressure of gallium, compared with the underlying GaAs substrate—any excess gallium will re-evaporate more slowly than the decomposition of the underlying GaAs substrate even for substrate temperatures as high as 650° C.